The present invention relates to a fluid flow metering technique, and particularly to a flow metering method and a flowmeter for metering an instantaneous flow rate or integrated flow rate of fluid flowing in a pipe.
A flowmeter is used to meter the flow rate of fluid such as kerosene, water, gas or the like consumed at home or business enterprise. A thermal type (particularly, indirectly heated type) flow sensor which is easily reduced in cost has been used as a flowmeter.
There is known such an indirectly heated type flow sensor that a sensor chip comprising a thin film heating element and a thin film temperature-sensing element which are laminated through an insulating layer on a substrate by using a thin film technique is disposed so that heat transfer is allowed between the sensor chip and fluid in a pipe (containing a fluid flow passage provided in a flowmeter so as to intercommunicate with an outer pipe). The temperature-sensing element is heated by making current flow into the heating element to vary the electrical characteristic, for example, the value of electrical resistance of the temperature-sensing element. The variation of the electrical resistance value (based on increase in temperature of the temperature-sensing element) is varied in accordance with the flow amount (flow rate) of fluid flowing in the pipe. This is because a part of the heating value of the heating element is transferred to the fluid, the heating value diffused into and absorbed by the fluid is varied in accordance with the flow amount (flow rate) to thereby vary the heating value to be supplied to the temperature-sensing element. Therefore, the electrical resistance value of the temperature-sensing element is varied. The variation of the electrical resistance value of the temperature-sensing element is made different in accordance with the temperature of the fluid. Therefore, there has been utilized such a manner that a temperature sensing element for temperature compensation is installed in an electrical circuit for measuring the variation of the electrical resistance value of the temperature-sensing element so that the variation of the metered flow-rate value due to the temperature of the fluid is suppressed to the minimum level.
Such an indirectly heated type flow sensor using a thin film element as described above is disclosed in JP-A-11-118566, for example. This flow sensor uses an electrical circuit containing a bridge circuit for achieving an electrical output corresponding to the flow rate of fluid.
In the flowmeter as described above, a fin plate for the heat exchange between the sensor chip and the fluid is projected into the fluid flow passage, and an electrical circuit portion containing a circuit board for calculating the flow rate, a display portion, a communication circuit connecting portion and other parts are disposed at the peripheral portion of the flow sensor, and all the function portions of the flowmeter containing the above parts are accommodated in the housing.
In a flowmeter using an indirectly heated type flow sensor, a part of the heating value occurring in the heating element (heater) is transmitted to the fluid, and on the basis of the fact that heat absorption is performed in accordance with the flow rate of the fluid, the output value of the electrical circuit which corresponds to the heat absorption amount is converted to the flow rate value by using a calibration curve. The calibration curve is achieved on the basis of an experiment or the like which was performed on fluid to be subjected to the flow-rate metering. Accordingly, when the fluid to be subjected to the flow rate metering has the same thermal property as the fluid used when the calibration curve was created, substantially no error occurs in the flow-rate metering when the conversion using the calibration curve is carried out.
However, when the target fluid which is to be subjected to the flow-rate metering is formed of a mixture containing plural kinds of molecules which are different in molecular weight such as kerosene, the fluid to be actually subjected to the flow-rate metering does not necessarily have the same thermal property as the fluid used when the calibration curve was created. That is, standards such as JIS or the like are provided to kerosene, however, such standards have some degree of permissible range for physical property values. As a matter of fact, as a result of investigation of kerosene which is actually sold in the market, it has been found that there is some dispersion in the thermal property thereof. This dispersion is caused by variation of compositions of kerosene due to the difference in oil factory in which kerosene is achieved from raw petroleum, etc.
As described above, some kinds of kerosene are different in thermal property because they are fluid belonging to the same category of kerosene, but have different compositions. Therefore, when fluid to be subjected to the flow-rate metering has a thermal property different from that of fluid used when the calibration curve is created, some error may occur in the metered flow-rate value when the conversion using the calibration curve is carried out in the flow-rate metering. Specifically, when a consumption amount of kerosene is metered by a flowmeter equipped to a kerosene consumer such as each home or the like, kerosene to be supplied does not necessarily have a fixed composition and a fixed thermal property. In some cases, an amount different from the actually-consumed kerosene amount is achieved as a metered flow-rate value.
Not only kerosene, but also gasoline, naphtha, diesel oil, liquefied petroleum gas (LPG), solution having concentration variation, slurry, sol or the like is cited as an example of fluid which is targeted for the flow-rate metering and has variation in composition causing variation in thermal property as described above.
Therefore, the present invention has an object to perform flow-rate metering having little metering error even when the thermal property of fluid is varied due to variation of the composition of fluid or the like. Particularly, the present invention has an object to provide a method and a flowmeter for the flow-rate metering as described above.
According to the present invention, in order to attain the above object, there is provided a method of metering a flow rate of fluid by making the fluid under metering flow into a metering flow path, achieving the electrical output corresponding to a metered flow rate of the fluid under metering in the metering flow path by an electrical circuit constructed so as to contain a thermal type flow sensor disposed in the metering flow path and converting the electrical output corresponding to the metered flow rate to a flow-rate value by using pre-created calibration curves, wherein:
the fluid under metering is filled in a reference flow path so as to freely flow, and the electrical output corresponding to a reference flow rate of the fluid under metering in the reference flow path is achieved by an electrical circuit constructed so as to contain a thermal type reference flow sensor disposed in the reference flow path;
the calibration curves contain reference flow-rate calibration curves and metered flow rate calibration curves that are achieved with reference to plural temperatures for reference fluid; and
on the basis of the electrical output corresponding to the reference flow rate achieved for the fluid under metering, a deviation amount of the temperature on the reference flow-rate calibration curve corresponding to the electrical output corresponding to the reference flow rate from a reference temperature is determined, a temperature correction amount to the metered flow rate calibration curve of the reference temperature is achieved on the basis of the deviation amount, and the conversion to the flow rate value of the fluid under metering is performed on the basis of the electrical output corresponding to the metered flow rate of the fluid under metering by using the metered flow rate calibration curve for the reference temperature and taking the temperature correction amount into consideration.
In an aspect of the flow rate metering method according to the present invention, with respect to fluid different from the reference fluid, a second temperature on a reference flow rate calibration curve which corresponds to the electrical output corresponding to a reference flow rate achieved at a first temperature is determined, a third temperature on a metered flow rate calibration curve at a certain flow rate corresponding to the electrical output corresponding to a metered flow rate achieved at the certain flow rate at the first temperature is determined, the ratio of the difference between the first temperature and the third temperature to the difference between the first temperature and the second temperature is achieved, and then the deviation amount is multiplied by the ratio to achieve the temperature correction amount.
In an aspect of the flow rate metering method according to the present invention, the calibration curves contain a specific gravity calibration curve indicating the relationship between the electrical output corresponding to the reference flow rate and the specific gravity, and conversion from the flow rate value of the fluid under metering to a volumetric flow rate value of the fluid under metering is performed by using the specific gravity calibration curve.
According to the present invention, in order to attain the above object, there is also provided a flowmeter for metering a flow rate of fluid passing through a housing by feeding the fluid into the housing, stocking the fluid in the housing and feeding out the fluid to the outside of the housing, comprising:
a metering flow path disposed in the housing for passing the fluid in connection with the feed-in or feed-out of the fluid into/from the housing; a reference flow path disposed in the housing, the fluid stocked being filled in the reference flow path so as to freely flow; a thermal type metering flow sensor disposed in the metering flow path; and a thermal type reference flow sensor disposed in the reference flow path, wherein the electrical output corresponding to a metered flow rate achieved by using the thermal type metering flow sensor is corrected on the basis of the electrical output corresponding to a reference flow rate achieved by using the thermal type reference flow sensor to thereby achieve a flow rate value.
In an aspect of the flowmeter according to the present invention, the flowmeter further comprises a metered flow rate detecting circuit constructed so as to contain the thermal type metering flow sensor, a reference flow rate detecting circuit constructed so as to contain the thermal type reference flow sensor, and a flow rate conversion circuit for performing conversion to the flow rate value corresponding to the electrical output corresponding to the metered flow rate achieved in the metered flow rate detecting circuit by using a reference flow rate calibration curve and a metered flow rate calibration curve which are achieved with respect to plural temperatures for a reference fluid, wherein the flow rate conversion circuit determines, on the basis of the electrical output corresponding to the reference flow rate achieved for the fluid under metering in the reference flow rate detecting circuit, a deviation amount of the temperature on the reference flow rate calibration curve corresponding to the electrical output corresponding to the reference flow rate from a reference temperature, achieves a temperature correction amount to the metered flow rate calibration curve of the reference temperature on the basis of the deviation amount, and converts to the flow rate value of the fluid under metering on the basis of the electrical output corresponding to the metered flow rate of the fluid under metering by using the metered flow rate calibration curve for the reference temperature and taking the temperature correction amount into consideration.
In an aspect of the flowmeter according to the present invention, with respect to fluid different from the reference fluid, the flow rate conversion circuit determines a second temperature on a reference flow rate calibration curve which corresponds to the electrical output corresponding to a reference flow rate achieved at a first temperature, determines a third temperature on the metered flow rate calibration curve at a certain flow rate corresponding to the electrical output corresponding to the metered flow rate achieved at the certain flow rate at the first temperature, and achieves the ratio of the difference between the first temperature and the third temperature to the difference between the first temperature and the second temperature, and the flow rate conversion circuit multiplies the deviation amount by the ratio to thereby achieve the temperature correction amount.
In an aspect of the flowmeter according to the present invention, the flow rate conversion circuit converts the flow rate value of the fluid under metering to a volumetric flow rate value of the fluid under metering by using a specific gravity calibration curve indicating the relationship between the electrical output corresponding to the reference flow rate and the specific gravity. In an aspect of the present invention, the thermal type metering flow sensor has a first fin plate projecting into the metering flow path for thermal interaction with the fluid, and the thermal type reference flow sensor has a second fin plate projecting into the reference flow path for thermal interaction with the fluid.
According to the present invention, in order to attain the above object, there is also provided a method of metering a flow rate of fluid by making the fluid under metering flow into a metering flow path, achieving the electrical output corresponding to a metered flow rate of the fluid under metering in said metering flow path by an electrical circuit constructed so as to contain a thermal type flow sensor disposed in the metering flow path by utilizing the fact that a heat absorption amount from the thermal type flow sensor to the fluid under metering corresponds to the metered flow rate of the fluid under metering in the metering flow path, and performing conversion to the flow rate value corresponding to the electrical output corresponding to the metered flow rate by using pre-created calibration curves, wherein:
the calibration curve is created for a reference fluid;
the fluid under metering is filled in a reference flow path so as to freely flow therethrough, and the electrical output corresponding to a reference flow rate of the fluid under metering in the reference flow path is achieved by an electrical circuit constructed so as to contain a thermal type reference flow sensor disposed in the reference flow path by utilizing the fact that a heat absorption amount from the thermal type reference flow sensor to the fluid under metering corresponds to the reference flow rate of the fluid under metering in the reference flow path; and
the output-corresponding value corresponding to the electrical output corresponding to the metered flow rate or the flow rate value achieved by the conversion using the calibration curve is corrected by using the electrical output corresponding to the reference flow rate, thereby correcting the error of the flow rate value due to the difference in thermal property of the fluid under metering from the reference fluid.
In an aspect of the flow rate metering method according to the present invention, the correction of the output-corresponding value corresponding to the electrical output corresponding to the metered flow rate is performed by subtracting, from the square value of the electrical output corresponding to the metered flow rate of the fluid under metering, a correction term of a result achieved by subtracting the square value of the electrical output corresponding to the reference flow rate achieved in advance for the reference fluid from the square value of the electrical output corresponding to the reference flow rate of the fluid under metering. In an aspect of the present invention, the temperature is measured, and the amount corresponding to a value at the measured temperature is used as the electrical output corresponding to the reference flow rate achieved in advance for the reference fluid when the correction term is achieved. In an aspect of the present invention, the correction term contains a correction coefficient for correcting the difference in characteristic between the thermal type flow sensor and the thermal type reference flow sensor.
In an aspect of the flow rate metering method according to the present invention, the correction of the flow rate value achieved by the conversion using the calibration curve is performed by multiplying the flow rate value achieved through the conversion by a correction parameter corresponding to the electrical output corresponding to the reference flow rate. In an aspect of the present invention, the temperature is measured, and the value at the measured temperature is used as the correction parameter.
According to the present invention, in order to attain the above object, there is also provided a flowmeter for metering a flow rate of fluid passing through a housing by feeding the fluid into the housing, stocking the fluid in the housing and feeding out the fluid to the outside of the housing, comprising:
a metering flow path disposed in the housing for passing the fluid in connection with the feed-in or feed-out of the fluid into/from the housing; a reference flow path disposed in the housing, the fluid stocked being filled in the reference flow path so as to freely flow; a first thermal type flow sensor disposed in the metering flow path; a second thermal type flow sensor disposed in the reference flow path; a first flow rate detection circuit constructed so as to contain the first thermal type flow sensor; and a second flow rate detection circuit constructed so as to contain the second thermal type flow sensor, wherein conversion from both of the first output-corresponding value corresponding to a first output value achieved in the first flow rate detection circuit on the basis of the thermal interaction between the fluid in said metering flow path and the first thermal type flow sensor and the second output-corresponding value corresponding to a second output value achieved in the second flow rate detection circuit on the basis of the thermal interaction between the fluid in the reference flow path and the second thermal type flow sensor to the flow rate value is performed on the basis of a correction output-corresponding value achieved by correcting the first output-corresponding value on the basis of the second output-corresponding value by using a calibration curve for reference fluid.
According to the present invention, in order to attain the above object, there is also provided a flowmeter for metering a flow rate of fluid passing through a housing by feeding the fluid into the housing, stocking the fluid in the housing and feeding out the fluid to the outside of the housing, comprising:
a metering flow path disposed in the housing for passing the fluid in connection with the feed-in or feed-out of the fluid into/from said housing; a reference flow path disposed in the housing, the fluid stocked being filled in the reference flow path so as to freely flow; a first thermal type flow sensor disposed in the metering flow path; a second thermal type flow sensor disposed in the reference flow path; a first flow rate detection circuit constructed so as to contain the first thermal type flow sensor; and a second flow rate detection circuit constructed so as to contain the second thermal type flow sensor, wherein conversion to a flow rate value is performed by using a calibration curve for reference fluid on the basis of a first output-corresponding value corresponding to a first output value achieved in the first flow rate detection circuit on the basis of the thermal interaction between the fluid in the metering flow path and the first thermal type flow sensor, and a corrected flow rate value is achieved by multiplying the flow rate value by a correction parameter corresponding to a second output value achieved in the second flow rate detection circuit on the basis of the thermal interaction between the fluid in said reference flow path and said second thermal type flow sensor.
In an aspect of the flowmeter according to the present invention, wherein the metering flow path and the reference flow path are formed in a flow rate metering portion disposed in the housing, an entrance of the metering flow path is formed to open toward a fluid reservoir portion formed in the housing so as to intercommunicate with a fluid inlet pipe formed in the housing, an exit of the metering flow path intercommunicates with a fluid outlet pipe formed in the housing, and an entrance and exit of the reference flow path are formed to open toward the fluid reservoir portion.
In an aspect of the flowmeter according to the present invention, the first thermal type sensor has a first fin plate projecting into the metering flow path for the thermal interaction with the fluid, and the second thermal type flow sensor has a second fin plate projecting into the reference flow path for the thermal interaction with the fluid.
The reason why the flow rate metering precision is not reduced by the present invention even when the composition of fluid under metering is different from that of reference fluid will be described below.
The following matters have been found as a result of examination in the difference in thermal property due to the difference in composition with respect to kerosene. That is, through the measurements of the dependence of thermal conductivity on kerosene composition (the deviation of the thermal conductivity of kerosene under metering from the thermal conductivity of reference kerosene), the dependence of specific heat on kerosene composition (the deviation of the specific heat of kerosene under metering from the specific heat of reference kerosene), the dependence of density on kerosene composition (the deviation of the density of kerosene under metering from the density of reference kerosene) and the dependence of kinetic viscosity on kerosene composition (the deviation of the kinetic viscosity of kerosene under metering from the kinetic viscosity of reference kerosene), it has been found that the dependence of kinetic viscosity on kerosene composition is more remarkable as compared with the thermal conductivity, the specific heat and the density. That is, the deviations of the thermal conductivity, the specific heat and the density are within about xc2x11% at maximum, however, the deviation of the kinetic viscosity is equal to xc2x110% or more.
Furthermore, it has been also found that the deviation of the kinetic viscosity affects the metered flow rate value of fluid under metering. According to the review of the magnitude of the effect, when a flow sensor having a heat-transfer member designed in a flat-plate shape so that fluid under metering flows along the flat plate as disclosed in JP-A-11-118566 is used, there are an effect of natural convection for the flow rate of zero and an effect of forced convection according to a theoretical analysis based on models. When the kinetic viscosity is represented by xcexd, the effect of the natural convection is proportional to xcexdxe2x88x92xc2xc, and the effect of the forced convection is proportional to xcexdxe2x88x92⅙. As described above, the effect of the natural convection is greater than the effect of the forced convection.
A contributory share of the convection to the flow rate value is not directly relevant to the actual flow rate of fluid flowing into a flow path, and thus it is expected to enhance the flow rate metering precision by removing the effect of the convection on the flow rate metering. The contributory part by the natural convection in the convection contributory share is independent on the flow in a metering flow path of fluid under metering, and it can be taken out as an output of a flow rate detection circuit in a reference flow path filled with the fluid capable of freely flowing as in the case of the flow rate metering in the metering flow path.
On the other hand, a contributory part by the forced convection is appended to the flow of the fluid under metering, and it is impossible to achieve the effect concerned separately from the flow. However, the effect of the contributory part by the forced convection is smaller than the effect of the contributory part by the natural convection, so that the flow rate metering precision can be enhanced by removing only the contributory part based on the natural convection through correction.
When an output voltage value achieved from a flow rate detection circuit containing a flow sensor disposed in the metering flow path is converted to a flow rate value, the relationship between the flow rate and the square value of the output voltage achieved by actual measurements for reference fluid (for example, kerosene serving as a reference) is used as a calibration curve. Accordingly, when the reference fluid is metered, the conversion to the flow rate value is accurately performed, however, when fluid different from the reference fluid is metered, a measurement error corresponding to the difference in thermal property between the fluid under metering and the reference fluid occurs. Therefore, according to the present invention, in order to detect the difference in thermal property of the fluid under metering from the reference fluid, a reference flow rate based on natural convection of the fluid under metering under a free-flowing state in the reference flow path is achieved. FIG. 21 is a graph showing variation of the relationship between the kinetic viscosity and the output voltage value of the flow rate detection circuit (reference flow rate detection circuit) containing the reference flow sensor due to the difference in kinds of kerosene. It is found that there is a great correlation between the kinetic viscosity and the output voltage of the reference flow rate detection circuit.
By using the output value of the reference flow rate detection circuit thus achieved, the output-corresponding value corresponding to the output voltage value of the flow rate detection circuit (metered flow rate detection circuit) containing the flow sensor in the metering flow path is corrected, or the flow rate value achieved by the conversion using the calibration curves created for the reference fluid is corrected, whereby the error of the flow rate value due to the difference in thermal property of the fluid under metering from the reference fluid can be corrected and thus the flow rate metering precision can be enhanced.